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cell

(sel ) [L. cella, a chamber] In the biological sciences, the basic unit of life. It is a group of self-sustaining biochemical reactions that are isolated from the environment by a selectively permeable lipid membrane. Among the key reactions are those that maintain a stable intracellular concentration of ions. For mammalian cells, typical internal concentrations include 140 mM K+, 5 to 15 mM Na+, 5 to 15 mM Cl-, and a pH of 7.2, which can be significantly different from their concentrations outside the cell. Other key reactions move molecules and molecular complexes within the cell, sometimes changing the shape of the cell. These reactions require energy; energy is generated by breaking apart hydrocarbon molecules by glycolysis and other characteristic intracellular metabolic reactions.SEE: glycolysis; SEE: metabolism; SEE: mitochondrion

STRUCTUREIntracellular chemical reactions are controlled by enzymes that are organized in organelles (stable molecular complexes). The polymer-based organelles include centrioles and the cytoskeleton; nucleic acid-based organelles include ribosomes; and membrane-enclosed organelles include the nucleus, endoplasmic reticula, Golgi complexes, lysosomes, peroxisomes, mitochondria, and storage and transport vesicles.

COMMON COMPONENTS AND ORGANELLES OF HUMAN CELLS Individual mammalian cells are usually microscopic, typically ranging from 5 to 50 µm in diameter. In humans, lymphocytes are small cells (6 µm in diameter), columnar epithelial cells (10 µm x 20 µm) are medium-size cells, and mature ova (120 to 150 µm) are some of the largest cells.

CELL DIVISIONIn mammals, all new cells arise from existing cells through cell division, and an animal's growth results largely from increases in the number of its cells, most of which differentiate into specialized cell types to form the body's various tissues. Cell division involves two major processes: karyokinesis, the division of the nucleus, and cytokinesis, the division of the remainder of the cell. When generating somatic daughter cells, karyokinesis uses a process called mitosis, which produces daughter cells with a full complement of chromosomes. When generating germ cells, karyokinesis includes a process called meiosis, which produces daughter cells with half the normal number of chromosomes. SEE: meiosis and SEE: mitosis for illus

acidophilic cell

acinar cell

adipose cell

adult stem cell

A precursor cell that can also give rise to identical precursor cells. Daughters of a stem cell can develop into a terminally differentiated cell type, or they can remain a stem cell. Adult stem cells are found in many tissues, such as bone marrow, brain, retina, skin, intestines, liver, testis, and pancreas. SYN: SEE: somatic stem cellSEE: embryonic stem cell

adventitial cell

A macrophage along a blood vessel, together with its undifferentiated perivascular cells.

air cell

An air-filled sinus cavity in a bone.

alpha cell

1. An enteroendocrine cell that produces glucagon and is found in the pancreatic islets. SYN: SEE: A cell2. An acidophil of the adenohypophysis. SEE: acidophil (2)

alveolar cell

1. Either of two types of epithelial cells lining the alveoli of the lung. Type I cells are simple, thin squamous epithelial cells. Type II cells, which are smaller and more numerous than Type I cells, secrete pulmonary surfactant.2. In the mammary glands, the milk-secreting epithelial cells, which are activated during lactation.

amacrine cell

A modified nerve cell in the retina that has dendrites but no axon. SEE: neuron

ameloblast cell

amine precursor uptake and decarboxylation cell

ABBR: APUD cell Any of the constituent cells of a diffuse neuroendocrine system, having metabolic pathways that make and utilize serotonin (5-HT). APUD cells include chromaffin cells, enterochromaffin cells, and certain cells found in the parathyroid gland, thyroid gland, pituitary gland, hypothalamus, and placenta.

amphophilic cell

anterior horn cell

A somatic motor neuron that has its cell body in the ventral (anterior) horn of the gray matter of the spinal cord. Its axon passes out through a ventral root and innervates skeletal muscle.

antibody-secreting cell

A B lymphocyte that differentiates after stimulation by an antigen and a presenting cell to produce and release large amounts of immunoglobulin.

antigen-presenting cell

ABBR: APC A cell that breaks down antigens and displays their fragments on surface receptors next to major histocompatibility complex molecules. This presentation is necessary for some T lymphocytes that are unable to recognize soluble antigens. Macrophages are the primary APCs, but B cells and dendritic cells also can act as APCs.SEE: T cell

argentaffin cell

A cell in the epithelium of the stomach, intestines, and appendix that secretes serotonin.

astroglial cell

atypical glandular cell

ABBR: AGCs A cell that is an abnormal finding on a Pap test. These cells are divided into “favor neoplasia” or “not otherwise specified” (NOS). NOS is further subdivided into endocervical or endometrial origin. Atypical endocervical cells are important because of their risk for significant disease. SYN: SEE: atypical glandular cell of undetermined significance

atypical glandular cell of undetermined significance

B cell

A lymphocyte that synthesizes and secretes antibodies. B cells originate and differentiate in the bone marrow and then populate the spleen, lymph nodes, and other lymphoid tissues. When exposed to an antigen, a B cell divides to form 1) plasma cells, which produce antigen-specific antibodies, and 2) a lesser number of memory B cells, which can quickly differentiate into plasma cells upon a second exposure to the original antigen. Antibody production is a key part of the humoral immune response of adaptive immunity. The humoral immune response is effective against bacteria, viruses, and other pathogens and provides the rationale for vaccination. SYN: SEE: B lymphocyteSEE: T cell

band cell

The developing leukocyte at a stage at which the nucleus is not segmented.

basal cell

1. A rounded or cuboidal epithelial stem cell found in the bottom layer of pseudostratified epithelia, such as the epidermis and the lining of the airways of the lung. 2. Either of two types of cell found in the bottom layer of the olfactory epithelium. SEE: basal cell proper; SEE: globose cell3. A rounded stem cell found in the taste buds and a progenitor of the specialized taste receptor cells.

basal cell proper

A flattened cell found in the bottom layer of the olfactory epithelium. SEE: basal cell (2)

basket cell

1. A smooth muscle cell found in some epithelia. It lies between glandular cells and the basal lamina of sweat, mammary, lacrimal, and salivary glands. SYN: SEE: myoepithelial cell2. Any of the granule cells found in the cerebral cortex. 3. Any of the small interneurons found in the outermost layer of the cerebellar cortex together with stellate cells. SEE: stellate cell

bone marrow cell

brush cell

burr cell

cancer cell

A cell present in a neoplasm and differentiated from normal tissue cells because of its degree of anaplasia, irregularity of shape, nuclear size, changes in the structure of the nucleus and cytoplasm, increased number of mitoses, and ability to metastasize.

columnar cell

columnar epithelial cell

cone cell

A cell in the retina whose scleral end forms a cone that serves as a light receptor. Vision in bright light, color vision, and acute vision depend on the function of the cones. SEE: rod cell

contrasuppressor cell

A T cell that inhibits the activity of suppressor T cells. A contrasuppressor cell shares this function with T helper cells, but it is distinguished from other CD4+ cells by its other cell surface markers and its unique group of cytokines. SEE: helper cell; SEE: T cell

cortical cell

A cell in the cortex of an organ, e.g., a neuron in the cerebral cortex.

cuboid cell

A cell, usually epithelial, with a height approx. equal to its width and depth.

cytotoxic cell

cytotoxic T cell

A CD8+ T lymphocyte that can destroy microorganisms directly through the release of perforin and proteolytic enzymes. These cells are particularly important in the defense against viruses, rejection of allografts, and, possibly, new malignant cells. SYN: SEE: cytotoxic cell; SEE: killer T cell

D cell

An enteroendocrine cell that produces somatostatin and is found in the pancreatic islets, stomach, and small intestine. SYN: SEE: delta cell; SEE: somatostatin cell

daughter cell

A cell formed by cell division.

decoy cell

A cell found in the urine with inclusion bodies in its nucleus. It indicates infection in renal transplant recipients.

delta cell

dendritic cell

An antigen-presenting cell that helps T cells respond to foreign antigens. Dendritic cells circulate in the blood and are also found in epithelial tissues, the Langerhans cells of the skin, and the interdigitating cells in lymph nodes.

effector B cell

embryonic stem cell

ABBR: ES cell A cell from the inner cell mass of the blastocyst that can give rise to all the somatic cells of the body. Embryonic stem cells can be maintained as pure stem cell cultures. SEE: adult stem cell

endothelial cell

Any of the epithelial cells that line blood vessels and lymph vessels. These cells are derived from mesenchyme cells of the embryo, are usually squamous, and form sheets one layer thick.SEE: endothelium

enterochromaffin cell

ABBR: EC cell An enteroendocrine cell that is found in the small intestine and produces serotonin. Enterochromaffin cells are similar to chromaffin cells.

enteroendocrine cell

Any of the scattered hormone-producing cells found in the pancreatic islets and throughout the gastrointestinal mucosa.

ependymal cell

Any of the epithelial cells that form a one-cell-thick layer lining the ventricles and the central canal of the central nervous system. The ventricular (apical) surfaces of many ependymal cells are covered with cilia or microvilli. Specialized regions of ependymal cells include the covering of the blood vessels and loose connective tissue of the choroid plexus. SYN: SEE: ependymoblast; SEE: ependymocyteSEE: choroid plexus

epithelial cell

Any of the cells forming the cellular sheets that cover surfaces inside and outside the body. Epithelial cells are closely packed and take on polyhedral shapes, from tall (columnar) through squat (cuboidal) to flat (squamous). Epithelial cells adhere strongly to one another, and one of their surfaces (the basal) sticks firmly to the basal lamina. SEE: epithelium

ethmoid cell

ethmoid air cell

eukaryotic cell

The type of cell composing multicellular and a number of unicellular organisms. Unlike prokaryotic cells, eukaryotic cells have many of their intracellular functions organized within organelles. SEE: organelle

F cell

An enteroendocrine cell that produces pancreatic polypeptide and is found in the pancreatic islets.

foam cell

follicle cell

follicular cell

1. The secretory cell of the thyroid gland. SEE: thyroid hormone2. Any of the flattened somatic cells that form a monolayer around each primary oocyte in the ovary. After puberty, when an oocyte matures, during a monthly cycle, its follicular cells divide, become cuboidal, and form a multilayered coating for the oocyte; at this stage, the follicular cells are called granulosa cells. SYN: SEE: follicle cell

folliculostellate cell

A supporting cell in the adenohypophysis. It produces bioactive peptides, including growth factors and cytokines.

foreign body giant cell

G cell

An enteroendocrine cell that is found in the stomach and produces gastrin.

ganglion cell

1. Any neuron whose cell body is located within a ganglion. 2. A neuron of the retina of the eye whose cell body lies in the ganglion cell layer. The axons of ganglion cells form the optic nerve.

germ cell

A cell whose function is to reproduce the organism. Early in development, primordial germ cells are found in the genital ridges of the embryo. Later, in the testis, the primordial germ cells are called spermatogonia, and in the ovary, they are called oogonia. When they mature, the germ cells, i.e., spermatogonia and oogonia, differentiate into haploid gametes, i.e., spermatozoa and ova.

giant cell

1. A multinucleated phagocyte created by several individual macrophages that have merged around a large pathogen or a substance resistant to destruction, such as a splinter or surgical suture. SEE: granuloma; SEE: tuberculosis2. A large multinucleated (40 to 60 nuclei) tumor cell characteristic of certain bone and tendon tumors. 3. A large multinucleated cell that invades the walls of the aorta and its major branches in giant cell arteritis. SYN: SEE: foreign body giant cell; SEE: multinucleated giant cell; SEE: syncytial giant cell

gitter cell

A macrophage present at sites of brain injury. The cells are packed with lipoid granules from phagocytosis of damaged brain cells.SEE: microglia

goblet cell

Golgi cell

graft facilitating cell

Any of a group of CD8+ T-cell receptor-negative cells that help donated bone marrow engraft in the recipient.

granule cell

1. Any of the small neurons that pack the granular cell layer of the cerebellar cortex, immediately below the Purkinje cell layer. Granule cells receive inputs from the spinal cord and brainstem. Axons of granule cells run perpendicular to the Purkinje cell dendrites, on which they synapse.2. Any of the neurons of the cerebral cortex that are not pyramidal cells. Cortical granule cells are categorized as spiny or nonspiny. 3. A small neuron that has no axons, found in the olfactory bulb.

granulosa cell

One of the many cuboidal cells that surround and nurture the maturing oocyte. SEE: follicular cell

gustatory cell

hair cell

An epithelial cell possessing stereocilia in the maculae, cristae ampullaris, and organ of Corti. These cells are receptors for the senses of position and hearing.

heart failure cell

A red lung macrophage often found in the sputum of patients with congestive heart failure. The red color is due to ingested red cells.

HeLa cell

A line of human epithelial cells that grows well in culture. It is an immortal cancer cell that has been maintained in continuous tissue cultures for decades from a patient with carcinoma of the cervix. It is named for the first two letters of the patient's first and last names, Henrietta Lacks. HeLa cells have been used in thousands of experiments on cell growth, differentiation, and cancer, and in virology, pharmacology, and other fields.

helmet cell

A schistocyte or fragmented blood cell, seen in hemolytic anemias.

HELMET CELLS (Orig. mag. ×640)

helper cell

A type of T lymphocyte whose surface is marked by CD4 receptors; it is involved in both cell-mediated and antibody-mediated immune responses. It secretes cytokines that stimulate the activity of B cells and other T cells and binds with class II histocompatibility antigens, which are processed by macrophages and other antigen-presenting cells. SYN: SEE: helper T cellSEE: antigen processing; SEE: T cell; SEE: cell-mediated immunity

Hürthle cell

hybridoma cell

hyperchromatic cell

A cell that contains more than the normal number of chromosomes and hence stains more densely.

I cell

An enteroendocrine cell that is found in the small intestine and produces cholecystokinin.

IgE-effector cell

Any of the white blood cells, including basophils and eosinophils, released during allergic reactions.

interdigitating cell

A type of antigen-presenting cell found in lymph nodes and lymphoid tissue.

interstitial cell

Any of the numerous cells found in connective tissue of the ovary, in the seminiferous tubules of the testes, and in the medulla and cortex of the kidney. The cells in the testes and ovaries produce hormones such as testosterone and estrogen.

interstitial cell of Cajal

Any of the nerve cells in the gastrointestinal tract that form pacemaking networks that allow normal peristalsis to occur. Diseases affecting the number or the functioning of these cells cause gastrointestinal motility disorders.

intestinal absorptive cell

In the small intestine, any of the tall columnar cells topped with a brush border made of microvilli.

islet cell

A cell of the islets of Langerhans of the pancreas.

juvenile cell

The early developmental form of a leukocyte.

juxtaglomerular cell

A modified smooth muscle cell in the wall of the afferent arteriole leading to a glomerulus of the kidney. This cell secretes renin when blood pressure decreases to activate the renin-angiotensin mechanism, which increases sodium retention, thus elevating the blood pressure.

K cell

An enteroendocrine cell that is found in the small intestine and produces glucose-dependent insulinotropic peptide.

lactotroph cell

Langerhans cell

LE cell

An abbreviation for lupus erythematosus (LE) cell, a polymorphonuclear leukocyte that contains the phagocytized nucleus of another cell. This cell may form when the blood of patients with systemic lupus erythematosus (SLE) is incubated and further processed according to protocol. The plasma of some patients contains an antibody to the nucleoprotein of leukocytes. These altered nuclei, which are swollen, pink, and homogeneous, are ingested by phagocytes, which are the LE cells. The ingested material, when stained properly, is lavender and displaces the nucleus of the phagocyte to the inner surface of the cell membrane. The LE cell phenomenon can be demonstrated in most patients with SLE but is not essential for diagnosis.

LE CELL (center) (Orig. mag. ×1000)

Leydig cell

littoral cell

A macrophage found in the sinuses of lymphatic tissue.

liver cell

Any of the cells that make up 70% to 85% of the liver's cytoplasmic mass. These cells are involved in synthesis and storage of protein; the transformation of carbohydrates; the synthesis of cholesterol, bile salts and phospholipids; and the detoxification, modification, and excretion of exogenous and endogenous substances. They also initiate the formation and secretion of bile.

lutein cell

A cell of the corpus luteum of the ovary that contains fatty yellowish granules. Granulose lutein cells are hypertrophied follicle cells that develop from the theca interna..

lymphoid cell

lymphokine-activated killer cell

ABBR: LAK cell Any of the natural killer cells obtained from the patient's blood and activated in culture with interleukin-2 LAK cells. The cells can then be used to treat patients with solid malignant tumors.

macroglial cell

marrow cell

mast cell

A large tissue cell resembling a basophil, essential for allergic and inflammatory reactions mediated by immunoglobulin E (IgE). Mast cells do not circulate in the blood although the substances they release may have systemic effects. Mast cells are present throughout the body in connective tissue, but are concentrated beneath the skin and the mucous membranes of the respiratory and digestive tracts. Mast cells are covered with IgE molecules, which bind with foreign antigens and stimulate degranulation, releasing such mediators as histamine, prostaglandins, leukotrienes, and proteinases from densely packed granules within the cytoplasm. These mediators produce type I (immediate) hypersensitivity reactions, e.g., urticaria, allergic rhinitis, asthma, angioedema, and systemic anaphylaxis.

mastoid cell

mastoid air cell

Any of the variable-sized, air-filled sinuses inside the mastoid process. About 20% of adult skulls have no mastoid air cells. SYN: SEE: mastoid cell

matrix stem cell

A stem cell derived from Wharton jelly.

memory cell

A cell derived from B or T lymphocytes that can quickly recognize a foreign antigen to which the body has been previously exposed. Memory T cells stimulate T helper lymphocytes and cytotoxic T cells; memory B cells stimulate the production of antigen-specific antibodies by B plasma cells. Both types of memory cells survive for years, providing a durable adaptive immune response against foreign antigens.

mesectodermal cell

mesenchyme cell

One of the two basic somatic cell lineages. The second somatic cell lineage is the epithelial. In contrast to epithelial cells, mesenchyme cells are not polarized and are frequently motile. In the early embryo, mesenchyme cells fill many of the spaces enclosed by epithelia. Later, mesenchyme cells will secrete the space-filling extracellular matrix molecules, such as collagen and glycoproteins, that characterize connective tissue.

mesenchymal stem cell

A stem cell found in connective tissue and capable of producing cells of the connective tissue lineages, such as cartilage, bone, muscle, and fat cells.

mesothelial cell

The type of epithelial cell that lines serous (pleural, peritoneal, and pericardial) cavities, blood vessels, and lymph vessels. These cells are usually squamous and form sheets (mesothelia) one layer thick. They are derived from mesenchyme cells of the embryo.SEE: mesothelium

microfold cell

ABBR: M cell 1. A cell in the gastrointestinal epithelium, covering patches of lymphoid tissue. M cells transport antigens from the intestinal lumen to the underlying lymphoid tissues for recognition and processing.2. An APUD cell that produces melanotropin and is found in the pituitary gland.

microglial cell

A small glial cell of the central nervous system and retina. Microglia have spiky branched processes and are arranged homogeneously throughout the brain and spinal cord. They are activated by disease and injury, after which they become phagocytic and sometimes resume their embryonic motility, like a macrophage.

mitral cell

One of the two principal neurons of the olfactory bulb. The other principal neuron of the olfactory bulb is the tufted cell. In a glomerulus, the mitral and tufted cells receive synaptic inputs from axons of the olfactory nerve. The axons of the mitral and tufted cells form the olfactory tract and synapse in the olfactory cortex.

mossy cell

An astrocyte or other glial cell with many branching processes. SEE: glial cell; SEE: neuroglia

mother cell

A cell that gives rise to similar cells through fission or budding. SYN: SEE: parent cell

mucosal cell

Any cell in a mucosal epithelium.

mucous cell

An epithelial cell that secretes mucus and IgA antibodies. Mucous cells and serous cells are the two varieties of secretory cells found in exocrine glands.

nerve cell

neural crest cell

Any of the embryonic cells of the neuron-glia lineage that form along the neural folds and migrate into the developing organism to produce a variety of tissues. The migratory ability of these embryonic epithelial cells is similar to the motility of mesenchyme cells and has led neural crest cells to be called mesectodermal cells. In the neural lineage, neural crest cells give rise to the dorsal root ganglia, the placodes that will develop into the olfactory and auditory sensory organs, the pituitary gland, the peripheral autonomic nervous system, and the neurenteric and APUD cells. In the glial lineage, neural crest cells give rise to Schwann cells and other peripheral satellite cells. In addition, neural crest cells of the cranial region give rise to certain facial connective tissue, including the bones of the nasal cavities, the roof of the mouth, and the sella turcica. SYN: SEE: mesectodermal cell

neuroglial cell

Niemann-Pick cell

nonstem cell

Any cell found in the bone marrow that cannot reconstitute the marrow or give rise to more differentiated blood cells.

null cell

1. A large lymphocyte without the cell markers of either a T cell or a B cell. Natural killer cells are examples of null cells.2. A cell within a pituitary adenoma that neither produces nor secretes hormones.

osteoprogenitor cell

oxyntic cell

parabasal cell

An abnormal but not malignant cell seen in some cytologic specimens obtained during Pap tests. It is found in women with vaginal atrophy, some postpartum women, some women suffering from anorexia or starvation, and some who have used progesterone for contraception.

phalangeal cell

One of the cells supporting the hair cells of the organ of Corti. These cells form several rows of Deiters cells (outer phalangeal cells) and a single row of inner phalangeal cells. SEE: Deiters cell (under Deiters, Otto F.C.)

phantom cell

pigment cell

pillar cell

One of two groups of cells (inner and outer) resting on the basement membrane of the organ of Corti in which elongated bodies (pillars) develop. These enclose the inner tunnel (Corti tunnel).

plasma cell

A cell derived from a B lymphocyte that has been sensitized to a specific foreign antigen and produces antibodies to that particular antigen. It may be found in the blood or in tissue fluid. SYN: SEE: effector B cell

plasmacytoid dendritic cell

ABBR: pDC Any of a subset of dendritic cells that, after contact with malignant cells or pathogens, migrate to lymph nodes and lymphoid organs, secrete interferons and other regulatory cytokines, and stimulate cellular immune responses by T lymphocytes. Overactivation of pDCs is commonly identified in autoimmune diseases like systemic lupus erythematosus and systemic sclerosis.

primordial cell

primordial germ cell

progenitor cell

prokaryotic cell

A cell composing many primitive unicellular organisms, such as bacteria. Prokaryotic cells do not have nuclei, which are partitioned by an intracellular membrane; instead the DNA forms one main coil in the cell cytoplasm.

Purkinje cell

pus cell

A leukocyte present in pus. Pus cells are often degenerated or necrotic.

pyramidal cell

A large common neuron found in the cerebral cortex. Pyramidal cells are flask-shaped or triangular and, in the parts of the cortex with six layers, occupy the fifth layer. Pyramidal cell dendrites project up into the most superficial layer of the cortex, while pyramidal cell axons run in the opposite direction, i.e., downward and out of the cortex.

radial glial cell

A structural macroglial cell that is a key component of the developing nervous system. Radial glial cells first appear in the neural tube, where their cell bodies are suspended between two thin cell processes. The apical process attaches to the inner (ventricular) surface of the neural tube, and the basal process attaches to the outer (pial) surface. Early in development, neuroblasts migrate radially along the scaffolding formed by the radial glial cell processes, and growing axons may follow the scaffolding longitudinally. Later, many radial glial cells retract their processes and differentiate into astrocytes.

regulatory B cell

Renshaw cell

resting cell

1. A cell that is not dividing. SEE: interphase2. A cell not performing its normal function, e.g., a nerve cell that is not conducting an impulse or a muscle cell that is not contracting.

reticular cell

1. An undifferentiated cell of the spleen, bone marrow, or lymphatic tissue that can develop into one of several types of connective tissue cells or into a macrophage. 2. A cell of reticular connective tissue. SEE: reticular tissue

Rouget cell

S cell

An enteroendocrine cell that is found in the small intestine and produces secretin.

satellite cell

1. A stem cell associated with skeletal muscle that may form a limited number of new muscle cells after injury. 2. One of the neuroglia cells enclosing the cell bodies of sensory neurons in spinal ganglia. SYN: SEE: amphicyte; SEE: capsule cell

segmented cell

selenoid cell

sensory cell

A cell that when stimulated gives rise to nerve impulses that are conveyed to the central nervous system.

septal cell

A type II alveolar cell that is adjacent to a septum of the alveoli and secretes pulmonary surfactant.

serous cell

An epithelial cell that secretes a watery fluid containing proteins, glycoproteins, and often antibodies (IgA, IgG, and IgM). Serous cells and mucous cells are the two varieties of secretory cells found in exocrine glands.

sympathochromaffin cell

syncytial giant cell

T cell

A lymphocyte that responds to specific antigens, with the assistance of antigen-presenting cells (APCs). T cells arise in the bone marrow and migrate to the thymus gland, where they mature; then they circulate between blood and lymph, serving as one of the primary cells of the adaptive immune response. Immature T cells are called thymocytes. Mature T cells are antigen specific. Their surface receptors (T cell receptors) respond only to a single antigen. T cells are further categorized using another family of surface protein markers called clusters of differentiation (CDs). All T cells have a CD3 marker. Additional markers differentiate the subclasses of T cells. CD4 T helper cells serve primarily as regulators, secreting cytokines that stimulate the activities of other white blood cells. CD8 T cells (cytotoxic T cells) directly lyse (kill) organisms, an important defense against viruses; most CD8 T cells also produce gamma interferon (INFγ), one of the strongest stimulators of macrophage activity. SYN: SEE: T lymphocyteSEE: immune response; SEE: lymphocyte; SEE: immunological surveillance; SEE: T-cell receptor; SEE: thymocyteA T cell can only recognize the “foreignness” of antigens after they have been modified by macrophages and other APCs. After this, T cells dominate the adaptive immune response by mobilizing B cells and other T cells of the cell-mediated immune pathways. T cells are responsible for type IV hypersensitivity reactions, such as graft rejection, and for tumor cell recognition and destruction.

target cell

1. An erythrocyte with a dark rounded central area surrounded by a lightly stained clear ring, which in turn is surrounded by a dense ring of peripheral cytoplasm. It is present in certain blood disorders, such as thalassemia, and in patients who have no spleen. SEE: hemoglobin C disease for illus SYN: SEE: codocyte; SEE: leptocyte2. The cell at which a signal, e.g., hormone or nerve impulse, is aimed.

TARGET CELLS In hemoglobin C disease (×600)

tart cell

A phagocyte that has ingested the unaltered nuclei of cells. These nuclei can be observed unchanged within the phagocytes.

taste cell

Any of the neuroepithelial cells within a taste bud that are receptors for the sense of taste. Each cell possesses on the free surface a short gustatory hair that projects through the inner taste pore. SYN: SEE: gustatory cell; SEE: taste receptor cell

tear-drop cell

tendon cell

terminally differentiated cell

A cell sufficiently committed to a particular function that it can no longer divide.

thymic epithelial cell

The epithelial cells that form the internal scaffolding of the thymus. These cells vary in shape and size but generally align in sheets and cords, partitioning the thymus into islands of close-packed lymphocytes in the organ's cortex. Thymic epithelial cells are not simply structural: they interact actively with adjacent lymphocytes.

totipotent cell

Touton giant cell

transitional cell

The stretchable epithelial cells that compose the transitional epithelium, which lines most of the urinary tract. Transitional cells are strongly interconnected. They are cuboidal when not under pressure, and they become flattened and squamous when stretched. Transitional epithelia are four to six cells thick, and the top transitional cells on the lumenal surface fuse to become larger and polyploid.

trophoblast cell

Any of the epithelial cells forming the surface of the spherical blastocyst stage embryo. Trophoblast cells are destined to give rise to many of the extraembryonic tissues.

white cell

white blood cell

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